Full color led module having integrated driver transistors

ABSTRACT

LED modules are disclosed having a control MOSFET, or other transistor, in series with an LED. In one embodiment, a MOSFET wafer is bonded to an LED wafer and singulated to form thousands of active 3-terminal LED modules with the same footprint as a single LED. Despite the different forward voltages of red, green, and blue LEDs, RGB modules may be connected in parallel and their control voltages staggered at 60 Hz or greater to generate a single perceived color, such as white. The RGB modules may be connected in a panel for general illumination or for a color display. A single dielectric layer in a panel may encapsulate all the RGB modules to form a compact and inexpensive panel. Various addressing techniques are described for both a color display and a lighting panel. Various circuits are described for reducing the sensitivity of the LED to variations in input voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/737,672, filed Jan. 9, 2013, by Bradley S. Oraw, assigned to thepresent assignee.

FIELD OF THE INVENTION

This invention relates to light emitting diodes (LEDs) and, inparticular, to a single die containing active circuitry in series withan LED to control current through the LED.

BACKGROUND

LEDs are typically formed as dies having an anode terminal and a cathodeterminal. An LED die is typically mounted on a larger substrate for heatdissipation and packaging. The substrate may contain additionalcircuitry, such as a passive electrostatic discharge device. The LED dieand optional substrate are then typically packaged, where the packagehas robust anode and cathode leads for being soldered to a printedcircuit board (PCB).

LEDs may be controlled by a current source to achieve a desiredbrightness. The current source may be a MOSFET or a bipolar transistorformed in a separate die. The current source and LED are typicallyconnected together by wires or a PCB.

Providing the current source separate from the LED die requires extraspace and interconnections, adding cost. Other disadvantages exist,including the possibility of mismatching components. It would bedesirable to provide a very compact LED module with an integratedcurrent source driver circuit.

Additional problems arise when driving multi-colored LEDs, such as in acolor display or for creating a white light source. An LED is a twoterminal electrical device with non-linear voltage versus currentcharacteristics. Below a particular voltage threshold, the LED is highimpedance. Above the threshold, the LED's impedance is much lower. Thisthreshold depends primarily on the bandgap of the semiconductor LED. Thebandgap is selected for a particular peak emission wavelength. Red LEDshave bandgaps on the order of 2 eV, blue LEDs have bandgaps on the orderof 3 eV, and green LEDs have bandgaps between 2 eV-3 eV. Since theforward voltage is directly related to the bandgap energy, red, green,and blue LEDs cannot simply be connected in parallel to output a desiredcolor or light; each color LED must have its own driver circuit. Thedifferent materials (e.g., GaAs, GaN, etc.) used to form the differentcolor LEDs also affect the forward voltages. Further, even within LEDsoutputting the same wavelength, their forward voltages vary due toprocess variations, so even connecting the same color LEDs in parallelis problematic. Providing a separate driver circuit for each LED andinterconnecting it to the LED adds space and cost. This is particularlyproblematic when trying to minimize the size of an RGB pixel in adisplay.

LEDs can be organized in passive matrix addressable arrays. Forinstance, a set of LEDs can be connected with their cathodes connectedto a row select driver and their anodes connected to a column data bus.Several of these rows can be used to form a larger array addressable byrow and column. Providing a controlled current through an addressedrow-column will energize the LED(s) at the addressed location(s) to emitthe desired color and intensity of light, such as for a color pixel in adisplay. Since the interconnection between the LEDs is non-zeroimpedance, the voltage drop throughout the interconnect network caninadvertently forward bias a non-addressed set of LEDs. Such incidentalforward bias will cause excess light in non-addressed segments, whichreduces light-to-dark contrast of the array.

This problem is made worse by placement of downward facing printed LEDs,where the downward facing LEDs are for reverse bias transient voltagesuppression. The downward LEDs are anti-parallel to the upward facingLEDs. In a simple addressing scheme, only the upward LEDs are intendedto emit light. When a row is not selected, the associated LEDs arebiased at a sub-threshold voltage or possibly reversed biased.Anti-parallel downward LEDs are problematic if the un-selected rows arereversed biased, which forward biases the downward LEDs causing them toemit light, reducing the light-to-dark contrast of the array.

It would be desirable to create integrated LED modules that avoid theabove-mentioned problems when connected in an addressable array.

It would also be desirable to create integrated LED modules where LEDsof different colors can be connected in parallel to form a high densityof compact RGB pixels.

It would also be desirable to create integrated LED modules of differentcolors that can be inexpensively packaged together in a single panel forgenerating light for backlighting, for general illumination, or for acolor display.

It would also be desirable to create an interconnection and addressingscheme for multiple LED modules to form a compact light or displaypanel.

SUMMARY

Problems related to parallel and addressable connections of LEDs, suchas in a color display, can be resolved by using active LED modules. Inone embodiment, a single vertical LED module includes an LED in serieswith a vertical drive transistor (a voltage-to-current converter). Threeterminals are provided on the module: a positive voltage terminal, anegative voltage terminal, and a control terminal for controlling thecurrent through the LED. The difference between the voltages applied tothe positive and negative voltage terminals must be sufficient toenergize the LED to its full desired brightness when the controlterminal is supplied a maximum value control signal.

The control terminal may be connected to the gate or source of a MOSFETconnected in series with the LED. The control terminal is added so thatthe threshold non-linearity of the LED impedance is actively, ratherthan passively, controlled. For an LED module where voltage is providedacross the power terminals of the module, the low impedance state (wherethe LED is emitting light) is determined by the control voltage appliedto the control terminal. Such an active LED in a parallel or addressablenetwork of LEDs would always be in a high impedance state until thecontrol signal activates the low impedance state. This active impedancecontrol reduces sensitivity to forward voltage and parasitic voltagedrops and reverse current paths.

In one example, red, green, and blue LED modules are connected inparallel in an array for a multi-color display, where any set of RGBLEDs (forming a single pixel) is addressable by applying the samevoltage across the voltage terminals of the three modules. The controlterminal of each module is connected to a different variable controlvoltage to achieve the desired brightnesses of the red, green, and blueLEDs in the pixel. The control voltages are applied in sequence at 60 Hzor greater so that the different forward voltages of the RGB LEDs are nolonger relevant.

In another embodiment, modules are connected in series and parallel fora white light source, where the white point is set by the relativecombination of red, green, and blue light. The control voltage for eachcolor and the duty cycle for each color are set to achieve the desiredwhite point.

In other embodiments, various circuits are integrated with the LED tomake the brightness of the LED less sensitive to variations in inputvoltage.

The modules may be formed by bonding an LED wafer to a driver transistorwafer, thereby connecting a terminal of each LED to a terminal of eachdriver transistor to form a series connection. The bonded wafers arethen singulated to form thousands of modules at a time. In anotherembodiment, the LED and driver transistor are grown over each other asepitaxial layers, or the driver transistor may be formed by diffusion orimplantation of dopants. The modules are extremely compact since thefootprint may be approximately the same as a single conventional LED die(e.g., 0.5 mm²-1 mm²).

In one embodiment, the LEDs are screen printed on a wafer. PrintableLEDs may be formed with a top surface area range of between 50-5000 um²,allowing the modules to have the same top surface area.

In a large lighting system using hundreds of medium power LEDs, it wouldbe impractical to provide a conventional drive circuit for each of theLEDs. For such white light sources, many LEDs are typically connected inseries, and a high voltage is connected across the string. In the priorart, providing such a high voltage sometimes requires a step upregulator, adding cost to the system. The present invention inherentlyprovides each LED with its own driver, allowing many LEDs, even ofdifferent colors, to be connected in parallel so that they may be drivenwith a low voltage (e.g., 5 volts). Providing each LED with its owndriver also enables each LED to be controlled to output a desiredbrightness despite process variations, changes in brightness withtemperature, and changes in brightness with age.

Various module embodiments are described along with various addressablearrays of LED modules that are suitable for LED displays or white lightsources.

In one embodiment, the packaging for the module is formed by printing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a single LED module in accordance with oneembodiment of the invention.

FIG. 2 is a cross-sectional view of a small portion of an LED waferbeing bonded to a driver transistor wafer.

FIG. 3 is a simplified cross-sectional view of a single singulatedmodule.

FIG. 4 illustrates various ways to apply fixed voltages and variablecontrol voltages to the three terminals of the module in FIG. 3,depending on the position of the LED and the type of driver transistorused.

FIGS. 5 and 6 illustrate the LED and driver transistor being formed bygrowing epitaxial layers.

FIG. 7 illustrates a singulated module die after packaging, such as in apanel.

FIG. 8 illustrates a PMOS driver transistor connected to the anode of anLED.

FIG. 9 illustrates a pnp bipolar driver transistor connected to theanode of an LED.

FIG. 10 illustrates an NMOS driver transistor connected to the anode ofan LED.

FIG. 11 illustrates an npn bipolar driver transistor connected to theanode of an LED.

FIG. 12 illustrates a PMOS driver transistor connected to the cathode ofan LED.

FIG. 13 illustrates a pnp bipolar driver transistor connected to thecathode of an LED.

FIG. 14 illustrates an NMOS driver transistor connected to the cathodeof an LED.

FIG. 15 illustrates an npn bipolar driver transistor connected to thecathode of an LED.

FIG. 16 illustrates an anti-parallel arrangement of LEDs used fortransient voltage suppression.

FIG. 17 illustrates RGB LED modules connected in parallel for a colordisplay or a white light.

FIG. 18 illustrates how the RGB LEDs in FIG. 17 may be sequenced usingthe control voltage to create any color, including white light.

FIG. 19 illustrates separate RGB LED modules packaged together, such asin a color display.

FIG. 20 illustrates an addressable network of RGB LED modules, using rowand column addressing.

FIG. 21 illustrates an addressing scheme for an array of RGB LEDmodules.

FIG. 22 illustrates a packaging configuration for RGB LED modules foruse in the network of FIG. 21.

FIG. 23 illustrates a zig-zag arrangement of color LED modules forimproved color mixing.

FIGS. 24-38 illustrate various 2-terminal LED modules.

FIG. 24 illustrates a resistor in series with an LED for setting acurrent through the LED.

FIG. 25 illustrates a variable resistor in series with an LED foradjusting a current through the LED.

FIG. 26 illustrates a voltage clamp or regulator in series with an LED.

FIG. 27 illustrates a current limiter or regulator in series with anLED.

FIGS. 28, 29, and 30 illustrate different voltage clamps across the LED.

FIGS. 31 and 32 illustrate current regulators and clamps being formedusing transistors and resistors.

FIGS. 33 and 34 illustrate current regulators and clamps being formedusing transistors, resistors, and diodes.

FIG. 35 illustrates a current source using two transistors.

FIG. 36 illustrates a voltage source using two transistors.

FIG. 37 illustrates a series current source using two transistors.

FIG. 38 illustrates 2-terminal RGB LED modules connected in parallel fora color pixel, including a white light pixel.

Elements that are the same or similar in the figures are labeled withthe same numeral.

DETAILED DESCRIPTION

FIG. 1 illustrates the circuitry in a 3-terminal LED module 10. Themodule 10 is formed as a single die, singulated from a wafer. The module10 contains an LED 12 and a PMOS driver transistor 14 with its sourceand drain in series with the LED 12 to control the current through theLED 12. The drain-source impedance adds to the impedance of the LED 12.Therefore, the total series impedance can be control by modulating thegate of the transistor 14. In this manner the transistor 14 performs avariable resistance or switching behavior. As such, forward current canonly flow if the gate is biased beyond the PMOS transistor turn-onthreshold. The module 10 may be packaged to have only three terminals16, 17, and 18.

Other configurations of a single transistor, active LED are possible, asdescribed later. The selection of a particular configuration of the LEDand transistor and the particular type of transistor depends on thecontrol requirements or constraints of the application.

FIG. 2 illustrates one embodiment of the structure of the module 10.FIG. 2 illustrates small portions of two wafers 20 and 21, which may beformed using different materials and technologies.

Wafer 20 is an LED wafer containing thousands of vertical LEDs. For bluelight, the material system may be AlInGaN, where the stoichiometrydetermines the peak emission wavelength. Forming such LEDs is wellknown. The blue LED may be eventually covered with a phosphor to produceany color. The LED wafer 20 may instead use other material systems toachieve a wide range of peak wavelengths from green to red. The LEDs mayinclude a multi-well active layer, as is well known. The wafer 20 isshown very simplistically since forming LEDs is conventional. Basically,an n-type epitaxial layer 23 and a p-type epitaxial layer 24 are grownover a growth substrate (e.g., sapphire, SiC, or GaAs). Light isgenerated at the pn interface. An active layer may be formed at theinterface. The growth substrate should be removed if highly resistive orlight absorbing. The n-type epitaxial layer 23 may also be thinned.

In one embodiment, the bottom surface of the LED wafer 20 is coated witha transparent conductor layer, such as a thin gold layer, to make ohmiccontact to the layer 23 and spread current. Each LED portion has atleast one metal electrode forming terminal T1. The metal electrodes maybe formed as thin fingers, asterisk-shaped, or otherwise take up a smallarea so as not to block a significant amount of light in the downwarddirection. In another embodiment, the bottom surface of the LED wafer 20is coated with a reflector layer so that light is only emitted from thesides or top of each singulated LED.

The bottom layer 26 in FIG. 2 represents any form of bottom conductor,including those described above. In the example, the bottom conductor isa cathode conductor, but in some embodiments, the bottom conductor is ananode conductor.

The top surface of the LED wafer 20 is prepared for being bonded to thebottom surface of the wafer 21 to form a substantially ohmic contact. Inone embodiment, the top surface of wafer 20 is a very flat reflectivemetal layer 28 that is bonded to a similar metal layer 30 on wafer 21 bypressure and heat. In another embodiment, the joining surfaces of thewafers 20 and 21 may be by a proprietary process performed by Ziptronix,Inc., such as described in U.S. Pat. No. 7,842,540, incorporated hereinby reference. The LED wafer 20 may have any diameter, such as 3-8inches. A suitable voltage applied between the top and bottom surfacesof the LED wafer 20 will cause the LEDs to emit light.

The top wafer 21 forms vertical p-channel MOSFETs associated with eachLED portion in the LED wafer 20. There will typically be thousands ofLEDs and MOSFETs formed in a wafer. The wafer 21 may use a p-typesilicon substrate in which is formed, by conventional photolithographictechniques, a p-type drain well 32, an n-type gate 34, and a p-typesource 36. Each drain well 32 may have a square shape coinciding withthe singulation edges of the modules.

The various dielectric layers and metal electrodes over the wafer 21 maybe formed by printing or by using conventional vacuum chambertechniques. If printing is used, such as screen printing, a dielectriclayer 38 is formed with openings over the gate 34 and source 36. A firstmetal layer 40 is then printed, using a screen for masking, in theopenings to contact the gate 34 and source 36. The first metal layer 40may be an ink containing metal (e.g., Ni, Ti, Al, etc.) particles and asolvent. When the ink is cured, the solvent evaporates and the metalparticles are sintered together. Another dielectric layer 42 is printedwith openings over the source 36 metal and gate 34 metal. An additionalmetal layer 44, such as aluminum, is printed over the source 36 metal,followed by a thick source electrode layer 46. The metal layers mayinclude a barrier layer. The terminals T2 and T3 in FIG. 2 are designedfor a particular type of packaging and array of modules, describedlater. The terminals T2 and T3 may be designed differently depending onthe application and packaging.

The various dielectric and metal layers over the wafer 21 may be formedafter the wafers 20 and 21 are ohmically bonded together to avoid damageto the conductors.

Printable LEDs may be formed with a top surface area range of between50-5000 um², allowing the modules to have the same top surface area. Forvery small LED sizes, etching is the preferred method for singulation.

The bonded wafers 20 and 21 are then singulated using conventionaltechniques such as etching, sawing, scribing-and-breaking, laser, etc.

FIG. 3 illustrates a simplified singulated LED module 10. In oneembodiment, the size (footprint) of the module 10 is about 0.1 mm²-1mm². The terminal T1 is shown taking up a small portion of the bottomsurface of the module 10 to allow light to escape from the bottomsurface.

To control the module 10 of FIG. 3 to emit light, assuming theconfiguration of FIG. 1, a positive voltage is applied to the sourceterminal T3, a negative voltage is applied to the cathode terminal T1,and a gate-source voltage (Vgs) exceeding the MOSFETs threshold isapplied to the gate terminal T2. In one embodiment, to forward bias theLED, the voltage differential across terminals T3 and T1 is greater than2 volts. For a blue LED 12, the required voltage differential may begreater than 4 volts.

FIG. 4 identifies various ways to control an LED module, depending onthe position of the LED and the type of MOSFET used. For example,instead of controlling the MOSFET by controlling its gate voltage, thegate voltage may be fixed (positive) and the source voltage may becontrolled to achieve the desired Vgs. Other configurations of LEDs andcurrent/voltage controlling transistors are shown in FIGS. 8-15,described later.

An advantage to using wafer bonding to bond the LED portion to thetransistor portion is that different materials (e.g., Si and GaN) forthe two wafers may be used. In the event that the LED and the transistorcan be based on the same material (e.g., GaN or GaAs), the LED layersand transistor layers may be epitaxially grown over the same growthsubstrate. In one embodiment, a well-known type of GaAs or GaN-basedtransistor, known as a high electron mobility transistor (HEMT), aheterostructure FET (HFET), a metal-semiconductor FET (MESFET), or amodulation-doped FET (MODFET), is grown over a growth substrate (e.g.,GaAs, GaN, SiC, sapphire, etc.) following or followed by an AlInGaN orAlInGaN LED generating blue to red light. The growth substrate may beremoved if highly resistive or absorbs light.

FIG. 5 illustrates an example where the growth substrate 50 has grownover it the LED layers 52, followed by the transistor layers 54. Thedielectric and metal layers over the top surface of the LED module 56may be similar to those layers in FIG. 2. If the substrate 50 isconductive, such as SiC, it may be left on the module. Light may exiteither the top surface, the bottom surface, or the side surface,depending on the materials and the application.

FIG. 6 illustrates an example where a growth substrate has grown over itthe LED layers 52 and the transistor layers 54. The growth substrate isthen removed and a metal electrode (not shown) is formed over theexposed surface of the LED layers 52. The dielectric and metal layersover the transistor may be similar to those layers in FIG. 2. Light thenmay exit from the bottom surface of the LED layers 52 opposite to thetransistor surface.

FIG. 6 may also illustrate an example where the transistor layers 54 aregrown on a substrate wafer and the LED layers 52 are then grown over thetransistor layers 54. The transistor layers 54 thus act as the growthsubstrate for the LED layers 52. In one embodiment, the growth substratemay be a conventional substrate for growing GaN layers, such assapphire, SiC, GaN, silicon, etc. The transistor layers 54 may be one ormore GaN layers for a FET, as described above. The LED layers 52 arethen grown to create a conventional GaN-based heterojunction LED thatemits blue light. The growth substrate is then removed, such as by usinglaser lift off or grinding to expose the transistor layers 54. Thetransistor layers 54 may then be thinned. In one embodiment, thetransistor layers 54 are n-type GaN layers, and, after the substrate isremoved, the n-type surface of the transistor layers 54 is thensubjected to conventional photolithographic masking and doping processes(e.g., by diffusion or implantation) to form a p-type gate region andn-type source regions shown in FIG. 2. The dielectric and metal layersshown in FIG. 2 may then be printed to create the transistor structureshown in FIG. 2.

In another embodiment represented by FIG. 6, the transistor is aheterojunction type and the layers 54 may be grown as an n-type sourcelayer, a p-type gate layer, and an n-type drain layer. Oppositeconductivities may be used. The LED layers 52 are then grown over thetop layer of the transistor layers 54. After the growth substrate isremoved, the semiconductor layers may be etched and metal layersdeposited to electrically contact the various layers in the transistor.The dielectric and metal layers shown in FIG. 2 may then be printed tocomplete the FET structure. Other types of transistors may be formedintegral with the LED layers 52. Forming GaN-based transistors isconventional.

The resulting wafer is then singulated to form many thousands ofindividual LED modules 10 at a low cost.

By growing the LED layers 52 and transistor layers 54 to form anintegral structure, any voltage drop across a bonded barrier (like inFIG. 2) is avoided and efficiency is improved. Fabrication cost is alsomuch less compared to the bonded structure of FIG. 2.

FIG. 7 illustrates the module 10 packaged to encapsulate it and toprovide conductors for applying power and control signals to the module10. The encapsulated module 10 may form part of a display panel in whichmany modules are encapsulated in the same panel. In FIG. 7, a substrate62 is provided, such as a transparent plastic or glass panel, with ametal conductor 64 for direct bonding to the terminal T1 of the LEDmodule 10. In a panel, there may be many conductors 64 connected tovarious LED modules in an array, or a single conductor sheet may connectthe LED modules in parallel. The metal conductor 64 is ultimatelyconnected to a power terminal. Light from the LED may be emitteddownward through the substrate 62. The metal conductor 64 may have ametal pad for bonding to the terminal T1. The metal conductor 64 mayalso include a transparent conductor portion. A dielectric layer 66 isthen screen printed over the substrate 62 to encapsulate the sides ofthe module 10. The dielectric layer 66 may also encapsulate othermodules supported by the substrate 62.

The module 10 may have a reflective film 68 formed on its sides prior toencapsulation to prevent side light emission, or the dielectric layer 66may be reflective, such as white. The film 68 may also represent adielectric coating if needed. Alternatively, side light from the LEDs isreflected upward and downward by the dielectric layer 66, such as wherethe dielectric layer 66 contains white titanium oxide particles. In sucha case, the substrate 62 may be reflective so all light ultimately exitsthrough the top surface of the panel.

A second metal conductor 70 is formed over the MOSFET and dielectric 66to contact the gate terminal T2. A dielectric layer 72 is formed overthe metal conductor 70, and a third metal conductor 74 is formed overthe dielectric layer 72 to contact the source terminal T3. In oneembodiment, the metal conductors 64, 70, and 72 are narrow column androw lines of an addressable LED panel, such as a color display or awhite light source.

In most cases, the dielectric layer 66 will be much thicker than thedielectric layer 72. The thin dielectric layer 72 is suitable forseparating the conductors 70 and 74 if the conductors 70 and 74 conductthe positive and control voltages for a PMOS transistor, since leakagebetween these two conductors would not be a concern. Thus, the terminalT1 should be the negative voltage terminal. The selection of which ofterminals T2 or T3 should be the positive voltage terminal and whichshould be the control terminal depends on the application. Typically thetop conductor 74 will be lower resistivity than the middle conductor 70.As such, a good choice for terminal T3 would be the higher currentpositive voltage terminal.

The panel may include many thousands of LED modules 10 of variouscolors, such as the primary red, green, and blue, or other colors, suchas yellow and white. All LEDs may be blue LEDs, with the red and greencolors being formed by red and green phosphors. If the panel is a whitelight panel to be used for general illumination or as a backlight for anLCD, each LED may be a blue LED coated with a phosphor that adds greenand red components to form white light. The panel may be on the order of2 mm thick and be any size. The various LEDs may be connected in anyconfiguration, such as series, parallel, or a combination to achieve thedesired voltage drop and current.

Light may be emitted from the packaged module 10 in various ways. If thetransistor wafer 21 is transparent to visible light, the conductors 70and 74 are transparent or narrow, and the bonded interface between thewafers 20 and 21 is transparent, the LED light may be emitted throughthe top surface in the orientation of FIG. 7. A transparent wafer 21 maybe SiC or GaN, and the transistor may be a well-known GaN HEMT, MOSFET,or MESFET. The bottom conductor 64 and the substrate 62 may bereflective.

Alternatively, the light may be emitted through the bottom of thepackage, where the conductor 64 is thin or transparent and the substrate62 is transparent. The wafer bonding interface may be a reflectivemetal.

Alternatively, all LED light may be transmitted through the side wallsof the LED, then reflected upward or downward through the top or bottomsurface of the package. The wafer bonding interface may be a reflectivemetal. The dielectric layer 66 may be diffusively reflective to reflectthe light upward and downward. The conductors 70 and 74 may be narrow ortransparent if light is to be emitted through the top surface. Theconductor 64 and substrate 62 may then be reflective. For bottom surfacetransmission, the conductors 70 and 74 may be reflective, the conductor64 is narrow or transparent, and the substrate 62 is transparent.

In a module (such as the single die module of FIG. 3), the controltransistor may be connected as a high side transistor or a low sidetransistor, and the transistor may be a MOSFET, a bipolar transistor, orany of the other types of transistors mentioned herein. All of thetransistor types are formed as vertical transistors. FIGS. 8-15illustrate some of the possible configurations. Forming all of thevertical transistor types is well-known.

FIG. 8 is identical to FIG. 1.

FIG. 9 uses a high side pnp bipolar transistor as the controltransistor.

FIG. 10 uses a high side n-channel MOSFET as the control transistor.

FIG. 11 uses a high side npn bipolar transistor as the controltransistor.

FIG. 12 uses a low side p-channel MOSFET as the control transistor.

FIG. 13 uses a low side pnp bipolar transistor as the controltransistor.

FIG. 14 uses a low side n-channel MOSFET as the control transistor.

FIG. 15 uses a low side npn bipolar transistor as the controltransistor.

The circuitry formed in the wafer 21 (FIG. 2) for each singulated LEDmodule may include multiple transistors and other components, such asresistors, interconnected in any manner. Each LED module may alsoinclude multiple LEDs interconnected with the components formed in thewafer 21. The interface bonding the LED wafer and the “electronics”wafer may include an electrode pattern that creates multiple conductivepaths between the LEDs and the components in the electronics wafer. Forexample, an electrode pattern formed on the top of the LED wafer maycorrespond to an electrode pattern formed on the bottom of theelectronics wafer for creating a mechanical bond and for providing acertain electronic interconnection. An adhesive may also be used toadditionally mechanically bond the wafers.

The issues involving a high density arrangement of LED modules employingthin dielectric layers and thin conductors may be complex. For example,the leakage between printed conductive layers will create parasiticcurrent paths that could either enhance or degrade controllability.Possible parasitic resistances due to leakage are illustrated in thecircuit of FIG. 16 as Rlk1, Rlk2, and Rlk3. The circuit 80 of FIG. 16may be a small portion of a display panel where anti-parallel connectedmodules are packaged close together to form a pixel for a single color.

The “upward facing” LED 82 is intended to be controlled to emit lightfor the display panel, and the “downward facing” LED 83 is intended toprovide reverse voltage transient protection of the LED 82 by shortingthe terminals 86 and 94 in the event of a high reverse transientvoltage. In a simple addressing scheme, only the LED 82 is intended tobe lit. When a row is not selected, the associated LEDs (e.g., LED 82)are biased at sub-threshold or possibly reversed biased. Theanti-parallel LEDs (e.g., LED 83) are problematic if the un-selectedrows are reversed biased, which forward biases those LEDs, causing themto emit light and reducing the light-to-dark contrast of the array.

In FIG. 16, the LED 82 emits light when the gate control terminal 84 isa threshold voltage below the positive voltage applied to terminal 86,so that the MOSFET 88 is on. At this time, the MOSFET 90 and LED 83 areoff. Resistance Rlk1 represents the leakage between the terminal 84 andthe positive voltage terminal 86. This leakage is beneficial since itprovides a weak pull-up for both p-channel MOSFETs 88 and 90, turningthe MOSFETs 88 and 90 off (and the LEDs 82 and 83 off) as a default,uncontrolled state. The leakage between positive voltage terminal 86 andthe negative voltage terminal 94, represented as Rlk2, causes power lossand hence lowers power efficiency; however, Rlk2 is a high resistance.The leakage between the control terminal 84 and the negative voltageterminal 94, represented as Rlk3, is a weak gate pull-down for MOSFET 88and thus degrades the controllability of the LED 82. Rlk3, however,causes some tracking between the gate and source of MOSFET 90 so it isbeneficial for turning off the LED 83.

As seen, issues of parasitic resistances should be taken into account inhigh density applications. Parasitic capacitances may also be taken intoaccount.

FIG. 17 illustrates circuitry in a single package containing at leastthree LED modules. The package may be a display panel containing anarray of addressable LEDs. One module includes an LED 98 that emits redlight, one module includes an LED 99 that emits green light, and onemodule includes an LED 100 that emits blue light. The LEDs 98 and 99 maybe phosphor coated blue LEDs. The modules include p-channel MOSFETs 101,102, and 103, similar to FIGS. 1 and 2. The package includes conductors106 (e.g., X-address lines) that electrically connect the sourcestogether and conductors 108 (X-address lines) that connect the LED'scathodes together so that the modules are connected in parallel. EachLED is controlled by a separate control voltage applied to the gate ofits respective MOSFET by conductors 110-112 (e.g., Y-address lines). Inthis way, any color light, including white, may be generated by thepackage. The three modules may form a single color pixel in a display ormay be part of a white light panel.

The advantage of the integrated modules, when controlling differentcolor LEDs connected in parallel, is that the modules can have twocommon terminals connected to the positive and negative voltages, withthe third terminal selecting a single LED at a time. By only turning onone color LED at a time, its forward voltage does not affect the voltageacross the other LEDs. For example, if the control voltages were allpulled low concurrently, the low forward voltage of the red LED 98 wouldprevent the green and blue LEDs from turning on. As long as only one LEDcolor is active at a time, then there is no conflict between differentforward voltages. The turn-on duration of the different LED colors canbe divided in time (time division multiplexing), and the control voltagelevel can be adjusted for the active LED forward voltage. In oneembodiment, the control voltages applied to the gates of the MOSFETs101-103 are provided sequentially at a frequency above about 60 Hz,where the relative duty cycles of the control voltages control theperceived color of light.

FIG. 18 is an example of the relative on-times of the red, green, andblue LEDs 98-100 in a single cycle for controlling the light emissionfrom the three modules. The control voltages may be different for eachcolor LED to cause the respective LED to emit a certain predeterminedflux level (e.g., a nominal maximum brightness), whereby any overallbrightness level and color, including white or neutral light, can beachieved by controlling the absolute on-times (for brightness) and therelative on-times (for color) per cycle.

FIG. 19 illustrates a package 108 containing three LED modules 109, 110,and 111. The package may be an entire panel of addressable LEDs, andFIG. 19 may just illustrate a small portion of the panel. Module 109contains a red LED, module 110 contains a green LED, and module 111contains a blue LED. In the example of FIG. 19, the cathode terminals T1of the LEDs are connected together by the conductor 114, supported bythe substrate 116. The direction of light emission from the package 108may be any of those directions discussed with respect to FIG. 7. Thetransistors in the modules 109-111 are p-channel MOSFETs, where a gatevoltage sufficiently below the source voltage turns on the transistorand LED. The gates of the transistors are connected in common by theconductor 118, and the sources of the transistors are separatelycontacted by conductors 120, 121, and 122, extending into and out of thedrawing page. The voltage across the conductors 114 and 118 is higherthan the forward voltage of any of the LEDs. By individually controllingthe source voltages in a time-division fashion, the respectivetransistors can be separately controlled to conduct any current tocontrol the mix of the RGB colors.

The dielectric layers 66 and 72 may be the same as in FIG. 7.

Alternatively, the sources of the transistors in FIG. 19 may beconnected together by a conductor replacing conductors 120-122, and thegates are separately contacted by conductors replacing the commonconductor 118 to allow individually controlling the transistors via thegate voltage.

In one embodiment, the structure of FIG. 19 represents a single 3-modulepackage with five terminals. In another embodiment, the structure ofFIG. 19 is only a portion of a much larger panel having a singlesubstrate 116, where each color pixel location contains the three RGBmodules. The dielectric 66 may be a single dielectric layerencapsulating all the modules on the panel. The pixels in a row may beaddressed by applying a voltage across row (X) conductors 114 and 118,and the individual LEDs at any pixel location in an addressed row may beturned on by applying a suitable control voltage to the column (Y)conductors 120-122. Many modules in a column may receive the samecontrol voltage, but LEDs in a non-addressed row will not turn on.

In high power (>0.1 W/in²) lighting applications (including backlightingan LCD) where many LEDs can be on at the same time, it is advantageousfor a given power to increase the operating voltage and reduce thecurrent. Power losses in the printed interconnects are proportional tothe square of the current; therefore efficiency can be increased byconnecting multiple LED segments in series, which sum to a largervoltage but lower current. FIG. 20 illustrates a light panel havingmultiple segments of parallel RGB LEDs (in rows) connected in series (incolumns). The panel may be much larger. Each combination of LED 124 andp-channel MOSFET 125 is a single module, formed in any of the mannersdescribed above.

In one example of the use of the panel of FIG. 20, white light iscreated as follows. A positive voltage (e.g., 15 volts) is applied tothe conductor 130 and a negative voltage (e.g., ground) is applied tothe conductor 132. Since the maximum forward voltage of any one of theLEDs is assumed to be 4 volts, and there are three LEDs in series, 15volts is sufficient to drive each string. In the example of FIG. 20, thered LEDs are in the leftmost column, the green LEDs are in the centercolumn, and the blue LEDs are in the rightmost column. All red LEDs inthe column are controlled by the same red control voltage applied to theconductor 136, all green LEDs in the column are controlled by the samegreen control voltage applied to the conductor 138, and all blue LEDs inthe column are controlled by the same blue control voltage applied tothe conductor 140. The control voltage magnitudes may be different toachieve the desired current through each column of LEDs. The controlvoltages are applied in sequence and at a duty cycle, such as shown inFIG. 18, in order to achieve the desired overall color output. Aresistor divider along the conductors 136, 138, and 140 causes eachMOSFET in a column to have the same Vgs. Another resistor dividerbetween conductors 132 and 130, formed of high value resistors, ensuresthat each row of modules has the same voltage across it when the LEDsare off so all MOSFETs in a column will turn on at the same time.

The light from the RGB LEDs will be mixed only a few millimeters fromthe face of the panel and/or a diffuser panel may be used to improve theuniformity of light.

Instead of using a resistor divider between conductors 130 and 132, aseparate voltage may be applied to each of the X conductors 130, 134,142, and 144 to apply 5 volts across each row.

Many small panels may be connected together to form a single largepanel. The small panels may be connected in any combination of seriesand parallel, depending on the desired voltages and currents, or eachpanel may be separately driven by its own power supply. In oneembodiment, the panel creates a 2×4 foot ceiling panel (a lamp) forgeneral illumination.

In another embodiment, the panel of FIG. 20 may be a color display. Fora color display, the resistor divider between the conductors 130 and 132is eliminated, and a single row of modules is addressed at a time byproviding a voltage of, for example, 5 volts, across the row. Then, acontrol voltage is applied to the conductors 136, 138, and 140 insequence to generate an RGB color for a single color pixel. The displaymay be any size.

If the panel of FIG. 20 is to be used for general lighting, there is noneed for row addressing, and the columns of series red, green, and blueLEDs are just addressed in a rapid time division repeating pattern byapplying control voltages to the conductors 136, 138, and 140 to turn onthe various MOSFETs. To the human eye, the colors blend together withoutflicker. Either the on-time per color, the particular number of LEDs ina series, or the control voltage per color may be selected to generatethe desired perceived color (e.g., white point). The emitted color maybe controlled to be selectable by the user.

For a lighting panel (as opposed to a color display with addressablepixels), convergence of the individual RGB elements is important toreduce visual nuisances of unmixed color. Therefore it is necessary topattern the individual LEDs colors in a regular pattern that willconverge into the desired color within a particular diffusion length.Secondly, for warm white colors, considerably more red power is neededthan green and blue. A two level RGB array having a regular pattern andtwice as many red LEDs as green and blue LEDs is shown in FIG. 21. FIG.21 illustrates an addressing scheme for the RGB LEDs in a lamp forgeneral illumination. The two level interconnect separates the redcontrol conductor from the green and blue conductors. Rows of blue andgreen LEDs alternate, while red LEDs are between the blue LEDs and greenLEDs in each row.

In FIG. 21, the cathodes of all the LEDs in the panel are connected to acommon ground conductor, and the gates of all the transistors (e.g.,p-channel MOSFETs) are connected to a common positive voltage conductor.Accordingly, the source voltages will be controlled to control thecurrent through the LEDs. A blue channel address bus 150 couples theblue X conductors 152 to the sources of the transistors for the blueLEDs. A green channel address bus 154 couples the green X conductors 156to the sources of the transistors for the green LEDs. The address busses150 and 154 may be connected together if the blue and green LEDs useapproximately the same forward voltage. A red channel address bus 158 iswide and is coupled to the sources of the transistors for the red LEDs.Accordingly, the overall color output by the panel is controlled by thevoltages applied to the three address busses 150, 154, and 158 and theduty cycles of the control voltages. An array of red, green, and blueLED modules 159 connected to the conductors and busses populates thepanel. The number and types of the red, green, and blue LEDs may beselected to achieve the optimal efficiency of the panel.

FIG. 22 shows a jagged cross-sectional view along line 22-22 in FIG. 21of a small portion of the two level RGB array panel of FIG. 21 showing aset of RGB LED packaged modules. All dielectric layers and conductorlayers may be formed by printing.

In FIG. 22, the substrate 160 may be a transparent plate. A ground (ornegative voltage) conductor 162 supported on the substrate 160 isconnected to the cathode terminals of the red module 166, the greenmodule 167, and the blue module 168. The conductor 162 may betransparent or thin to allow light to be emitted through the substrate160. A dielectric layer 164 encapsulates the sides of the modules166-168. A gate conductor 166 is connected to the gates of the MOSFETsin the modules 166-168. A fixed positive voltage (relative to theconductor 162 voltage) is applied to the gate conductor 166. Adielectric layer 168 is formed above the conductor 166. Separateconductors 152 and 156 (extending into and out of the drawing) areconnected to the respective source electrodes 157 of the blue and greenmodules 168 and 167 to separately control the currents to the blue andgreen LEDs. Each red LED module has a raised source contact 170 thatextends above the conductors 152 and 156. A dielectric layer 172 isformed over the conductors 152 and 156 and over the dielectric layer168. The red channel address bus 158 is then formed over the dielectriclayer 172 to contact all the sources 173 of the red LED modules 166. Thebus 158 covers the entire array of RGB LED modules and also acts as areflector. As seen, the resulting panel has two levels of controlconductors for separately controlling the currents to the red, green,and blue LEDs. The LEDs in the array of FIG. 21 are connected inparallel, and the different colors of LEDs are controlled in asequential manner, as previously described. Multiple arrays may beconnected together in series and parallel to achieve any size panelhaving any overall brightness with optimal voltages and currents.

In one embodiment, the conductors 152 and 156 are about 1 mm wide orless. The LEDs may produce pixel sizes of 50 um²-1 mm². For a whitelight source, where uniform light across the panel is desirable, the RGBcolors are mixed at a height only about 1-2 mm above the panel. Adiffuser sheet may also be used. The red, green, and blue LEDs may beenergized sequentially at 60 Hz or greater to avoid flicker. Therespective duty cycles determine the overall color emitted by the panel.

Alternatively, FIG. 22 may represent a single, packaged RGB lamp forminga single controllable pixel of a color display or for any otherapplication.

FIG. 23 illustrates a zig-zag arrangement of red, green, and blue LEDmodules for improved color mixing. In FIG. 23, short diagonal lines ofred LED modules 180 are arranged in a zig-zag column. Similarly, shortdiagonal lines of green LED modules 182 are arranged in a zig-zagcolumn, and short diagonal lines of blue LED modules 184 are arranged ina zig-zag column. An additional red LED module column may be insertedbetween the green and blue columns to add more warmth to the resultingwhite light. The zig-zagging of the RGB light better mixes the light fora more uniform white light across the panel. The connections to the RGBLED modules may be the same as described with respect to FIGS. 21 and 22so are not shown for simplicity.

In some applications, there is a benefit to connect LED modules of thesame color in parallel. There may be any number of legs connected inparallel. A string of LED modules may form each leg of the parallelcircuit, and each leg may include a different number of LED modules inseries. The LEDs in a single leg are energized together and each leg iscontrolled independently. This technique may be used to adjust theoverall brightness (flux) emitted from the parallel circuit, whileallowing the LEDs to be operated at maximum efficiency, which is usuallyobtained at a relatively low current. Therefore, to achieve a higherbrightness, instead of increasing current (resulting in lowerefficiency) through a string of LED modules, a string of LED moduleshaving fewer LEDs may be energized at the optimum current. Time divisionmultiplexing may be used to obtain any brightness level at a highefficiency.

FIGS. 24-38 illustrate various configurations of 2-terminal LED modulesthat may be formed as wafer-bonded modules, or where the passive oractive circuitry is epitaxially grown on the same wafer as the LED, orwhere the passive or active circuitry is formed by diffusing orimplanting dopants into the LED wafer. The modules may have a topelectrode and a bottom electrode, where the bottom electrode is thecathode of the LED, and the top electrode is an electrode of the passiveor active circuitry. Other orientations of the LED are envisioned. Thecircuits of FIGS. 24-38 control the current through the LED and/orprovide substantially uniform luminance of the LED by reducingsensitivity to input voltage variations.

FIG. 24 illustrates a resistor 190 in series with an LED 192, in asingle module die 194, for adjusting a current through the LED. Thesimplest V-to-I converter is realized with a series resistance, as inFIG. 24. The resistance buffers the variation of the LED voltage. For alarge input voltage and a relatively small forward voltage of the LED,the current is approximately equal to the input voltage divided by theseries resistance. If the input voltage is much larger than the forwardLED voltage, then a fixed resistance, as in FIG. 24, may be sufficientto reduce the uncertainty in the LED properties. For RBG LED modulesconnected in parallel, the series resistance in each module may beselected so that each LED is simultaneously illuminated. This preventsan LED with a low forward voltage, such as the red LED, clamping thevoltage across the green and blue LEDs to a voltage below the forwardvoltages of the green and blue LEDs. The series resistance drops asufficient voltage to prevent such clamping.

For input voltages close to the forward LED voltage, a variableresistance 196, such as shown in FIG. 25, is used in a 2-terminal moduleto adjust the current through the LED 192. A variable resistance is usedsince the forward voltage of the LED varies somewhat from LED to LED andthe precision of the resistance value is important to achieve therequired current. The variable resistance may be an active device,including a transistor.

FIG. 26 illustrates a voltage clamp or regulator 198 in series with oracross an LED 192 in a single module. The series resistor realizationdoes not reduce the luminance sensitivity to input voltage variations.To buffer the voltage source uncertainty, a voltage clamp or regulator198, or a current regulator or limiter 200, shown in FIG. 27, can beused. For sufficiently large input voltages, the luminance is thereforeindependent of the input voltage.

FIGS. 28, 29, and 30 illustrate different voltage clamps 202, 206, and210 across the LED. A voltage clamp can be realized by a single diode(FIG. 28), several diodes in series (FIG. 29), or a zener diode (FIG.30). The diode clamps limit the voltage applied to the LED, and theseries resistances limit the current to the LED.

A more robust means of voltage clamping and diode limiting can berealized using transistors. FIG. 31 illustrates a clamp 212 across theLED, and FIG. 32 illustrates a current limiter 214 formed usingtransistors and resistors. The transistors provide an active means tovary the series resistance and hence reduce sensitivity to inputvoltage. The module of FIG. 9 may be used to form the circuit of FIG. 31by forming a resistor in the semiconductor material connected betweenthe base of the transistor and the negative terminal. Similarly, themodule of FIG. 11 may be used to form the circuit of FIG. 32 by forminga resistor in the semiconductor material connected between the base ofthe transistor and the positive terminal.

Various other ones of the circuits of FIGS. 24-38 can be formed usingthe modules of FIGS. 9 and 11 by forming additional circuit elements inthe semiconductor material and forming connections between the elements.

FIG. 33 illustrates a clamp 218 and FIG. 34 illustrates a currentregulator 222 and clamp formed using transistors, resistors, and diodes.

FIG. 35 illustrates a current source 226 using two transistors. Betterregulation can be achieved if more transistors are used.

FIG. 36 illustrates a voltage source 230 using two transistors.

FIG. 37 illustrates a series current source 234 using two transistors.

FIG. 38 illustrates any of the 2-terminal modules of FIGS. 24-37connected in parallel, where the three modules 240, 242, 244 containred, green, and blue LEDs to form a single light element in a lightpanel, such as for general illumination or backlighting. The circuitry246 is set for each color LED to emit the desired brightness (by settinga certain current through the LED) while also setting the desiredvoltage drop across the module to allow each of RGB LEDs to turn on. Theintegrated LED modules can be paralleled to achieve uniform luminancewithout other external components. In another embodiment, all the LEDare the same color, including blue LEDs with a phosphor coating togenerate white light.

Any of the modules of FIGS. 24-38 may also include the transistorcontrollers of FIGS. 1-23 in series with the LED to form 3-terminalmodules.

The proposed solutions described herein integrate the V-to-I driver withthe LED in a single die. The driver and LED form an integrated circuit,which is fabricated on two wafer-bonded substrates or on the samesubstrate. This integration reduces intrinsic and parasitic uncertaintyof the LED and the interconnection to the global system. The integrationalso greatly reduces the size and cost of the circuit compared to usingnon-integrated V-to-I drivers. This allows each LED to have its owndedicated driver.

Additionally, providing each LED with its own controllable driverenables each LED to be controlled to output a desired brightness despiteprocess variations, changes in brightness with temperature, and changesin brightness with age.

The preceding examples have used MOSFETs and bipolar transistors;however, the scope of this invention is not limited by the transistortechnology. Realizations can be created using a CMOS, BiCMOS, BCD, orother integrated circuit processes. Additional transistor technologiesnot shown could be used as well such as JFET, IGBT, Thyristor (SCR),Triac, and others.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

1. A lighting device comprising: a first die containing a firsttransistor in series with a first light emitting diode (LED), whereinthe first LED emits a first color, wherein the first die has first anodeterminal, a first cathode terminal, and a first transistor controlterminal; a second die containing a second transistor in series with asecond LED, wherein the second LED emits a second color, wherein thesecond die has second anode terminal, a second cathode terminal, and asecond transistor control terminal; a third die containing a thirdtransistor in series with a third LED, wherein the third LED emits athird color, wherein the third die has third anode terminal, a thirdcathode terminal, and a third transistor control terminal; a firstconductor connected to the first anode terminal, the second anodeterminal, and the third anode terminal; a second conductor connected tothe first cathode terminal, the second cathode terminal, and the thirdcathode terminal, such that the first die, the second die, and the thirddie are connected in parallel; a first control conductor connected tothe first transistor control terminal for controlling a current throughthe first LED; a second control conductor connected to the secondtransistor control terminal for controlling a current through the secondLED; a third control conductor connected to the third transistor controlterminal for controlling a current through the third LED; and a controlcircuit for supplying a control signal to the first control conductor,the second control conductor, and the third control conductor forcontrolling respective brightnesses of the first LED, second LED, andthird LED to achieve a desired combined color emission.
 2. The device ofclaim 1 wherein the control circuit supplies a time division multiplexedcontrol signal to the first control conductor, the second controlconductor, and the third control conductor such that the first LED, thesecond LED, and the third LED are on at different times.
 3. The deviceof claim 2 wherein a duty cycle of an on-time of the first LED, thesecond LED, and the third LED determines a perceived combined coloremission.
 4. The device of claim 1 wherein the first LED emits redlight, the second LED emits blue light, and the third LED emits greenlight.
 5. The device of claim 1 wherein the first LED, the second LED,and the third LED have active layers that all emit blue light, wherein afirst wavelength conversion material causes the second LED to emit redlight, and wherein a second wavelength conversion material causes thethird LED to emit green light.
 6. The device of claim 1 wherein thefirst LED, the second LED, and the third LED have active layers thatemit blue light, red light, and green light, respectively.
 7. The deviceof claim 1 wherein the first LED, the second LED, and the third LED forma first component in an array of substantially identical components toform a panel.
 8. The device of claim 7 wherein the array ofsubstantially identical components form addressable color pixels in adisplay panel.
 9. The device of claim 7 wherein the array ofsubstantially identical components form an illuminating panel.
 10. Thedevice of claim 1 wherein the first die, the second die, and the thirddie are included in a panel for generating light that combines the firstcolor, the second color, and the third color.
 11. The device of claim 10wherein the panel contains a first plurality of dies substantiallyidentical to the first die, where the first plurality of dies areconnected in series across a first voltage, wherein the panel contains asecond plurality of dies substantially identical to the second die,where the second plurality of dies are connected in series across thefirst voltage, wherein the panel contains a third plurality of diessubstantially identical to the third die, where the third plurality ofdies are connected in series across the first voltage, wherein thecontrol circuit provides a first control signal coupled to all thetransistor control terminals of the first plurality of dies to control abrightness of the first plurality of dies, wherein the control circuitprovides a second control signal coupled to all the transistor controlterminals of the second plurality of dies to control a brightness of thesecond plurality of dies, and wherein the control circuit provides athird control signal coupled to all the transistor control terminals ofthe third plurality of dies to control a brightness of the thirdplurality of dies.
 12. The device of claim 11 wherein at least oneresistor is coupled between each of the transistor control terminals ofthe first plurality of dies, the second plurality of dies, and the thirdplurality of dies.
 13. The device of claim 11 wherein each of the firstplurality of dies, the second plurality of dies, and the third pluralityof dies are arranged in a zig-zag fashion in the panel to improve mixingof light.
 14. The device of claim 11 wherein the control circuitgenerates the first control signal, the second control signal, and thethird control signal in sequence in a cycle to control the contributionof the first color, the second color, and the third color to a perceivedcombined color.
 15. The device of claim 1 wherein the first die, seconddie, and third die are printed.